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How to raise my compression

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mr steve wayne 575

New member
What can I use to raise engine compression on a boat that has not been started for 5 years? My restoration project is:
1984 Sea Ray
350 Cubic inch, 4 barrow carb.
Serial: GMC 756889
Serial above starter on plate: 0756889
GM57LG (5GI
Hours 638.3
6 cylinders test out at 68 to 80 compression
2 cylinders test out at 125 compression
My question is should I use Marvel mystery oil, Penetrating oil, or something else.
Thanks
Mr Steve
 
Hurry and have a " leak down " test done on each cylinder.------Magic potions in a can will not work here in my opinion !!
 
Welcome. Did you do the compression test with all spark plugs out, coil power input unplugged, carb with open choke and at least half throttle, full battery charge, and your fingers crossed?
 
As for your choices, of what to use? Probably gonna be "something else"! If it sat awhile, it could be dry seized rings and/or some lite rust in valve seats, depending on where it was stored, and how it was prepared. We have a stern drive section here on the Forum. Racer, has more than enough experience to cover your questions, as so do I, but my Chevy V8 experience is more tailored to auto and trucks, as well as racing. Was the engine stuck? It certainly is low hours, shouldn't be worn out.
 
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for now it was done on dry compression test. I was wondering of what I could use to help free cylinders up and to see that they are soaked a few days before I did a wet compression. I think it may be 2 soon to do an air on test the cylinders and look for a leak down ( I am not that far along) unless they are oiled a little. I do not want to do any damage to cylinder walls. Motor turns over real good. Just wondering what I could put in cylinders to protect them and loosen things up.
I have been told Marvel Mystery oil (never used it) or Penetrating oil, or maybe something else. I had a car mechanic in my are tell me Mardall oil. Never heard of that one and neither has the internet.
So I am a little confused on what to use.
Thank You I appreciate your feed back.
Mr Steve
 
Chris, the compression ( listed in my beginning post) I am told it is 2 low to start the engine.
Timguy, the boat was purchased by me and my understanding it has been setting about 5 years. This seams to be true because the last time it was licensed was 5 years ago.
 
I would be temped to give each cylinder a dose of Seafoam Beep Creep... Let it sit for a couple days, turn it over a couple times and repeat the process. I would do it for about two week. If you had any stuck rings they would be freed up, if they were going too.
 
Some valves would be OPEN during extended storage.-----Faces may have rust on them.----Put some storage / fogging oil in each cylinder.-----Crank the motor over to get lube oil pressure.----Then hook up ignition for spark and start it.
 
After a little soaking, take Chris's advice and fire it up. The heat and movement may free any stuck rings, the valves have rotators on them so they may regain seal as well. Then retake compression test as mentioned earlier. Report back so we all can "go to school"! Thanks, Tim
 
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